1. Technical Field
The present disclosure relates to an integrated acoustic transducer in MEMS technology and to the manufacturing process, and in particular to a micro-electromechanical (MEMS) microphone of a capacitive type with a suspended-membrane mobile electrode and reduced residual stresses.
2. Description of the Related Art
As is known, an acoustic transducer, for example, a MEMS microphone, of a capacitive type generally comprises a mobile electrode, in the form of a diaphragm or membrane, arranged facing a fixed electrode, to provide the plates of a capacitor. The mobile electrode is generally anchored, by means of a perimetral portion, to a substrate, while a central portion is free to move or bend in response to a sound-wave pressure acting on a surface of the mobile electrode. Since the mobile electrode and the fixed electrode form the capacitor, bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of the capacitor. In use, said variation of capacitance is converted into an electrical signal, supplied as an output signal of the MEMS microphone.
As an alternative to MEMS microphones of a capacitive type, MEMS microphones are known, in which the movement of the membrane is detected by means of elements of a piezoresistive, piezoelectric, or optical type, or also exploiting the tunnel effect.
MEMS microphones of a known type are, however, subject to problems deriving from residual (compressive or tensile) stresses internal to the layer that forms the membrane. The factors that affect stress are multiple, and are due, for example, to the properties of the materials used, to the techniques of deposition of said materials, to the conditions (temperature, pressure, etc.) at which deposition is made, and to possible subsequent thermal treatments.
Residual stresses are frequently the cause of mechanical deformations of the membrane, such as warping or buckling, and can significantly affect the performance of the MEMS microphone by reducing the sensitivity.
Even though it is possible to control partially the amount of residual stress in the membrane by means of an appropriate design of the membrane itself and by evaluating the optimal manufacturing conditions, the result obtained is not satisfactory for applications in which a high sensitivity is required. In these cases, in fact, the mechanical behavior in response to sound-wave stresses is in any case dominated by the level of residual stress in the membrane.
To overcome these problems, described in WO 2008/103672 is a MEMS microphone of a capacitive type in which the mobile electrode (with membrane of a circular shape) is suspended over a cavity by means of a single anchorage element fixed with respect to a supporting beam provided in the same layer in which the fixed electrode is formed. The point of coupling of the anchorage element with the mobile electrode is located in the center of the membrane that forms the mobile electrode. In this way, the mobile electrode can release the residual stresses through free radial contractions or expansions.
However, membrane mobile electrodes suspended to the fixed electrode by means of a central anchorage are readily susceptible, during use, to undesirable modes of pitch and roll, which cause a degradation of the performance of the MEMS microphone that uses said mobile electrodes.